So for the benefit of those of us who never flew in FAA land: How is this rule kept in practice?
Can a dispatcher order a flight to not take less than a certain amount of fuel or define a maximum amount? And the captain, will he then have to live with this and can not order an amount of fuel outside of the scope the dispatcher defined (of course, keeping within all other limitations like weight, performance restrictions, tank volume or minimum block)? Or will Captain and Dispatcher have a brief chat before every flight and agree on a sensible amount that satisfies both of them?
And what happens when fuelling requirements change on short notice during a flying day, e. g. due to deteriorating weather before the 4th leg? Is the Captain allowed to let this influence the amount of extra fuel he wants on board without talking to his Dispatch first and convincing them of this change of plan? This of course applies in the opposite direction as well; say an airfield that showed nasty weather in the forecast opens up unexpectedly - is a reduction of the extra fuel within the Captains authority?
Also: Who is taking the blame if something untoward happens? Will both the Captain and the Dispatcher have to defend their actions in a court or are there differences between those two and the buck stops at the Captain only, seeing that the hypothetical incident was presumably preceded by a threat that was up to the flight crew to resolve?